Numerous important devices require for their operation a supply of electrons and, more specifically, electrons delivered in the form of a beam. For example, a Scanning Electron Microscope (SEM) uses a focused electron beam to examine objects and a cathode ray tube (CRT) type image display uses directed beams of electrons to illuminate its typically phosophor-containing pixels. Other displays, e.g., flat panel displays of the Field Emission Display (FED) type also use electron beams. Lithographic and metrologic machines also use electron beams. Yet other devices are used to amplify electron beams.
The prior art teaches devices capable of emitting electrons that can be delivered or focused in the form of a beam as well as devices which can amplify incident electron beams. These devices include cathodes, photocathodes, thermionic emission devices, vacuum tubes, field emission devices, semiconductor electron beam emitters and others. Recent requirements for high brightness (defined as current density per solid angle) and narrow energy spread of the electrons constituting the beam seriously limit the applicability of many of these devices. In particular, many applications such as high resolution SEM, high definition lithography as well as displays would benefit from robust high brightness source operating at 10 keV fields, or fields of higher and lower values, and yielding brightness values of 105 A/cm2/sterdian and an energy spread of less than 1 eV and preferably less than 0.1 eV.
Standard cathodes are not capable of delivering such high brightness beams with the requisite narrow energy spread because the electron emission conditions in a typical cathode are difficult to control, inherently noisy and the cathodes are not sufficiently robust. In the case of photocathodes, photocathodes that operate with available light sources and optics are very sensitive to contamination. Photocathodes which promise to be more robust lack available light sources delivering photons of apropriate energy to operate the photocathodes to produce such high brightness and narrow energy spread electron beams. Thermionic emission devices have lower brightness and larger energy spread. Field emission devices are difficult to control, noisy and also can not satisfy the robustness requirements in that they must be operated at very high vacuum to maintain a clean environment for the emitters. Another serious problem with field emission devices used in displays is that reverse ion bombardment damages the emitter. Another problem is that electric fields near breakdown limits are required for the desired current output in the attempt to increase brightness. Such large fields result in reduced lifetime of the emitter.
Electron beam emitters and amplifiers using semiconductor materials promise to deliver higher brightness electron beams. A semiconductor electron beam device operates on the principle that a semiconducting material will normally have a low electrical conductivity impeding the flow of an electrical signal therethrough. Electrical carriers, however, can be generated in the semiconductor material in response to electron bombardment. For example, in U.S. Pat. No. 5,592,053 Fox et al. recognize that diamond, which is a wide bandgap semiconductor, can be used as a target in an electron beam device and be made to conduct electrical current, thus acting as an amplifier. There are numerous additional references on the subject of using diamonds for electron emission, such as U.S. Pat. No. 4,993,033 to Lin, which teaches the use of diamond rendered conductive by an electron beam in a high power fast switch. In other words, the generation of electrical carriers, such as electron-hole pairs, under electron bombardment increases the conductivity of the semiconducting material allowing the passage of an electrical signal therethrough.
It is known that improved performance in electron emission from a semiconductor emitting surface can be obtained by a condition of negative electron affinity (NEA) at the emitting surface. Surfaces which exhibit NEA will promote the emission of electrons which are close to the emission surface. For more information on this subject of NEA and appropriate processing and doping of semiconductors for photocathodes the reader is referred to A. W. Baum, Doctoral Thesis, Stanford University Department of Applied Physics, Stanford, California 1997; and for diamonds in particular to L. Diedrich, et al., “Electron Emission and NEA from Differently Terminated, Doped and Oriented Diamond Surfaces”, Diamond and Related Materials, 8, 1999, pp. 743–747; R. Kalish, “Doping of Diamond”, Carbon, 37, 1999, pp. 781–785 further discusses appropriated doping of diamond and many other references. In fact, many of the above-described prior art devices use NEA to generate electron emission.
Noteworthy examples of the use of semiconductors in electron beam emitters and amplifiers include U.S. Pat. No. 5,680,008 to Brandes et al. which teaches the use of compact low-noise dynodes incorporating semiconductor secondary electron emitting materials to amplify a signal by amplifying an electron beam in numerous stages. The Brandes dynodes are essentially electron multipliers which amplify the signal generated by an incident particle. In U.S. Pat. No. 5,986,387 Niigaki et al. teach a transmission type electron multiplier and electron tube. This device has a high secondary electron generation efficiency and uses a thin film of diamond as the semiconductor. In U.S. Pat. Nos. 6,005,351; 6,060,839 and 6,100,639 Sverdrup, Jr. et al. teach the use of thin diamond semiconductors for secondary electron beam emission and their use in electron beam amplifiers as well as flat panel displays.
Unfortunately, despite all attempts, none of the above described sources and/or amplifiers of electron beams are capable of satisfying the requirements of high brightness, narrow energy spread and robustness at the same time. Specifically at electric fields of 10 keV a brightness of more than 105 A/cm2/sterdian and an energy spread of less than 1 eV and preferably less than 0.1 eV and robustness is not achievable with these prior art sources. Therefore, what is needed is a robust source which does not require a high vacuum for delivering an electron beam of high brightness and narrow energy spread.